Printing inks and printing methods may be broadly divided into three major classes which vary considerably in ink composition, method of application, and drying mechanism. Typographic printing is a method of printing from raised characters or plates which are first inked and then pressed against the surface to be printed, thus transferring the ink. The ink must have sufficient viscosity (tack) and adhesion so that it can be evenly distributed on the rollers of the press and be transferred properly from the type or printing plates to the paper or other substrate being printed.
The second major method of printing is lithography which is also known as planographic printing since the process prints from plane surfaces and depends on the fact that grease and water are mutually repellent. If a design is drawn with a greasy substance on a finely-grained metal surface which retains moisture, the lithographic ink will adhere to the design and not the remainder of the plate. In the form of offset lithography, the greasy image is formed on a thin grained metal plate by photographic means and the metal plate is curved and clamped around the plate cylinder of an offset press. Lithographic or offset inks are more viscous than typographic inks and must be substantially resistant to emulsification due to the high speeds developed by offset presses and the resulting high rates of shear. Additionally, the presence of large amounts of volatile solvents in the ink vehicle is undesirable since the solvent will tend to dissolve away the greasy image and cause the plate to go blind. Evaporation of the solvent further gives rise to increased viscosity of the ink which in turn affects the fidelity of printing.
The present invention is concerned with the third major method of printing, i.e., printing by the intaglio process in which, as in all forms of intaglio printing, the design to be printed is etched or engraved below the surface plane of the printing plate. The surface of the plate is then covered with ink to fill in the line or pockets of the etching or engraving and is thereafter wiped so as to not disturb the ink in the etching or engraving. Printing is accomplished by pressing the paper firmly against the plate to transfer the ink from the engraving to the paper. The ink formulation employed for intaglio printing depends upon the type of plate employed and the speed of operation.
One form of using the intaglio process is the engraved steel plate process used in machine printing of currency. The printing of currency from an engraved flat steel or copper plate requires inks that are greaseless, possess short consistencies, and dry by oxidation. Furthermore, since the paper employed is usually dampened, the inks must not bleed or run in water.
Another and extensively used form of the intaglio process is the rotogravure process used to print catalogs, magazines, and supplements to the Sunday newspaper, etc. The term "rotogravure" for this version of the intaglio process stems from the use of rotating rolls (cylinders) as the printing element. The process is often referred to as simply "gravure", and the two terms will be used herein interchangeably.
In rotogravure intaglio inks which are used to print at high speeds from chemically or electronically engraved copper or chromium faced rolls, the wiping of the plate is achieved mechanically by means of a doctor blade. Drying of rotogravure inks is accomplished by evaporation of the solvent. These inks are thin and contain pigments of low specific gravity which are suspended in a vehicle composed mainly of a resin and a solvent.
In the development of printing inks, particularly gravure or solvent inks, printing qualities such as color reproduction, retention of color tone, pigment wetting, and the like, have received special attention. Since gravure inks employ large amounts of volatile solvents for their application, special attention has been directed to avoiding environmental pollution caused by extremely toxic vapors such as from chlorinated hydrocarbons. In addition, gravure inks must be substantially free of particles that might become trapped between the doctor blade and the printing roll and cause continuous streaks to appear. Streaking due to this cause is not uncommon and may be largely corrected by filtering the ink to remove such particles before they become trapped, or by running a pointed bamboo stick under the doctor blade to dislodge the stuck particle.
A particular form of gravure printing is used for reproduction of colored images. Color reproduction by the gravure process, as with other printing processes, requires that the colored image be photographed at least twice and most often four times through appropriate different filters to produce so-called color separations. These color separations, after many photographic steps and incorporation of a suitable screen, are used to produce engraved cylinders, one for each color to be printed.
Although a wide range of hues may be produced with two colors, a more complete range is achieved with the three basic colors generically referred to as yellow, magenta, and cyan, but a black is often added, too. The toners used for rotogravure process inks are substantially transparent, and the printed image is formed by a subtractive process. Thus, the yellow is usually printed first and dried, and the red printed in exact register after the yellow. A beam of white light impinging on the printed paper at this stage of the formation of the print undergoes selective absorption of the blue and green components with reflection of the remainder.
After the cyan is printed, absorption of none, some, or substantially all of the spectral components of white light becomes possible, depending on the distribution of the inks and their densities. Thus, grays and blacks may be formed by three-color printing. However, it is very common practice to add a separate black cylinder and to regard black (also referred to as key) as the fourth color to allow the printer better control of the hues in the reproduction at the time of printing. Thus, four color printing is very commonly used for quality work, but often additional special colors may be used or the type may be printed separately. In skilled hands, these techniques are capable of yielding excellent color reproductions.
The gravure color process, because it requires overprinting one color by another at least once and often four times, makes particular demands on the cleanliness of the ink. The occurrence of streaking in any one of the inks, particularly the black, cyan, or magenta, materially degrades a high quality reproduction and makes it unacceptable. The usual remedies, such as filtering, often correct this problem.
In some instances, however, streaking occurs in gravure printing that is not correctable by simple measures. In at least some instances, such persistent streaking appears to be associated with an adherent deposit on the part of the printing roll that is not etched. In the past, unsuccessful efforts to eliminate streaking have included filtering the ink, changing ink batches, changing ink viscosities, changing ink suppliers, changing solvent blends, changing ink temperature, and repeatedly cleaning up the ink unit. Other efforts have included scrubbing, dechroming and rechroming the cylinder against which the ink rollers press, changing paper and drying temperatures, changing ink formulation, and varying the ink formulation from no wax to a high wax ink content. Even the removal of deposits that jam behind the doctor blade, changing the pressure and angle of the doctor blade, and changing doctor blade material have not satisfactorily eliminated the problem of streaking.
The present invention is concerned with a rotogravure printing ink composition and process in which the ink is applied at ambient temperature and is dried at above ambient temperature, i.e., at a temperature at least 20.degree. F. above ambient. The drying of these gravure printing inks occurs by evaporation of the volatile solvent component consisting essentially of hydrocarbons, oxygenated compounds, or mixtures thereof, each of which at atmospheric pressure has a boiling point below about 150.degree. C. The term "drying" as used herein means the loss of fluidity and stickiness when a film of the ink, not more than one thousandth of an inch thick when coated on glass, is exposed to air for not more than 30 minutes at 72.degree. F.
This invention is particularly concerned with inks for reproducing a colored image by the color rotogravure reproduction process, and such inks are herein designated "process gravure inks", said reproduction being effected by overprinting one color on another at least once. In all cases in which the term "gravure printing ink" is used herein, it is intended to refer to a composition used to reproduce a colored or a black and white image, which image may include text; and, also to a composition that dries by the evaporation of volatile solvent consisting of the components described above. The term "gravure printing ink" as used herein excludes compositions that are applied by a rotogravure apparatus to produce functional coatings, such as a continuous adhesive coating, or carbon paper coating.
It has been general practice not to include non-fugitive antioxidants in printing inks. For inks which dry by oxidation of a drying oil or oleoresinous film, such inclusion prevents drying. But because certain of such compositions may form a skin on storage or when on the press, a minute amount of fugitive antioxidant such as guaiacol (2-methoxyphenol), which boils at 205.degree. C, is sometimes used. Its evaporation after printing allows drying by oxidation to proceed. Since rotogravure inks do not form skins on storage, there has been no reason to incorporate antioxidants.
U.S. Pat. No. 3,375,120 to Remer discloses the use of antioxidants in compositions designed to be applied at the boiling point of the solvent for manufacture of functional coatings. The antioxidant is used to preserve the quality of the functional coatings during long-term storage or during long periods of exposure to outdoor conditions. U.S. Pat. No. 4,077,807 to Kramer et al discloses an ink composition for ball point pens, which is not a gravure ink as defined herein and is devoid of volatile solvent.